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Halogen-lithium exchange mechanism

If 3-bromothiophene is added to butyllithium at a sufficiently low rate, no 2-lithio-thiophene is formed, because the halogen-lithium exchange is much faster than the vicinal metallation of the bromo compound. Support for the mechanism of trans-metallation was provided by adding an equivalent amount of thiophene to a solution of 3-lithiothiophene in THF, cooled at —30 °C after 1.5 h only the 2-lithio derivative was present in the solution [9]. In Et20 this trans-metallation was very slow. [Pg.124]

Despite its apparent simplicity, the halogen-lithium exchange reaction remains mechanistically intriguing. - Depending on the substrate and conditions, halogen-lithium exchange may involve a four-center transition State, radical intermediates via a single-electron transfer (SET) mechanism, or a nucleophilic mechanism perhaps via an "ate"... [Pg.750]

Schlosser and co-workers have reported the shift of lithium in lithiated l-bromo-3-(tri-fluoromethyl)benzene 2,fa Quenching at — 100 C gives exclusively the product derived from 2, whereas after 2 hours at — 75 C, arene 2 is completely converted into less basic 3. A lithium-iodine exchange takes place in lithiated 2-chloro-3-iodo-6-(trifluoromethyl)pyridine 4, which at —85 C is totally converted into the less basic isomer 5.7 These rearrangements have been discussed in terms of a base-catalyzed halogen dance or halogen-shuffling mechanism. [Pg.232]

A reaction similar to the overall copper-halogen exchange observed on addition of perhaloarenes to lithium dimethylcuprate is also given by pentafluorobenzene, pentachlorobenzene, 2,3,5,6-tetrachloropyridine (146), and tetrafluorobenzenes (144). A small excess of methyllithium was again required, otherwise the reactions were very slow. Formation of methylcopper during the addition was not observed with the poly-fluoroarenes, but a mechanism was proposed similar to that for the perhaloarenes (146). Equation (22) represents the overall reaction for pentafluorobenzene. In most cases, methane was evolved nearly quantitatively. [Pg.225]

This mechanism has been confirmed using isotopically labelled fluorobenzene. Benzynes have been trapped by dienes in Diels-Alder reactions. They can also be generated by strong bases such as sodium amide in liquid ammonia in this way bromobenzene can be used without the interference of metal-halogen exchange which is the preferred course with lithium alkyls. [Pg.42]

The chemistry of alkenyl- and alkyl-substituted cyclopropylidenes has been explored. They were generated from a. gm-dihalide by lithium-halogen exchange, and were shown to give both allenes and 1,5-CH bond insertion reactions. A new mechanism for the insertion of carbenes into OH bonds was proposed on the basis of a computational study. A cyclic five-membered transition state, composed of the car-bene and two hydroxyl-containing molecules, was shown to be lower in energy than proton transfer or direct insertion into an OH bond. ... [Pg.270]


See other pages where Halogen-lithium exchange mechanism is mentioned: [Pg.516]    [Pg.611]    [Pg.123]    [Pg.611]    [Pg.203]    [Pg.250]    [Pg.234]    [Pg.114]    [Pg.418]    [Pg.240]    [Pg.216]    [Pg.24]    [Pg.216]    [Pg.216]    [Pg.126]    [Pg.216]    [Pg.10]    [Pg.436]    [Pg.828]    [Pg.830]    [Pg.373]    [Pg.423]    [Pg.21]    [Pg.1306]    [Pg.1520]    [Pg.460]    [Pg.1308]    [Pg.460]    [Pg.1308]    [Pg.281]    [Pg.206]    [Pg.1306]    [Pg.206]    [Pg.93]    [Pg.460]    [Pg.373]    [Pg.324]    [Pg.752]    [Pg.54]    [Pg.117]    [Pg.158]    [Pg.12]   
See also in sourсe #XX -- [ Pg.516 ]




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Halogen exchange

Lithium-halogen exchange

Mechanism halogenation

Mechanisms halogenations

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